The olfactory system is faced with the task of extracting biologically and socially relevant signals from the diversity of chemicals occurring in the environment. To accomplish this task, vertebrates, including humans, possess several morphological types of receptor neurons, the most obvious being the microvillous and ciliated types. The proposed studies test the hypothesis that anatomically different olfactory receptor neurons respond to different classes of odorants, express different odorant receptor molecules and connect to different sites in the olfactory bulb. This proposition will be studied in catfishes and goldfishes because these are the only vertebrates for which both feeding- related odorants and sex-signaling pheromones are chemically well- defined. Thus correlative physiological, anatomical and molecular methodologies can be used to test the hypothesis rigorously and directly. We already have shown that anatomically different receptor neurons project to specific target sites within the olfactory bulb and that different sites in the olfactory bulb respond preferentially to different classes of odorants. The first group of experiments will utilize electrophysiological and anatomical methods to test whether specific bulbar areas respond preferentially to one of the known classes of natural odorants: amino acids, nucleotides, bile salts or sex pheromones; and whether any of these areas are targeted uniquely by microvillous or ciliated receptor cells. The second group of experiments will examine which molecular features of the odorant molecules are key to the specificity of the bulbar responses. The final group of experiments will use combined retrograde tracing and in situ hybridization to test whether the anatomically and physiologically different types of receptor neurons express different olfactory receptor molecules. Use of multiple methodologies permits a systems level analysis of the olfactory representation of biologically-important chemical stimuli from the level of molecular receptor to neuronal assemblages. These studies will determine whether, in vertebrates, specific types of olfactory receptor neurons and receptor molecules mediate specific odorant-driven behaviors such as feeding, courting behavior or even subconscious sexual hormonal responses to specific odor signals.